ABSTRACT. Adverse effects of tributyltin (TBT) chloride were investigated on the reproductive system in male adult rats as exposed during puberty. Fifty Sprague-Dawley rats at the age of 35 days were assigned to five different groups: negative control receiving vehicle, methyltestosterone (10 mg/kg B.W.), and TBT chloride treatments (5, 10, and 20 mg/kg B.W.). Animals were treated by oral gavage for ten consecutive days and sacrificed at 5 weeks after final treatment. The treatment of TBT chloride at the high dose of 20 mg/kg B. W. significantly decreased homogenization-resistant testicular sperm counts (p<0.05). The TBT chloride treatment at the doses of 10 and 20 mg/kg B.W. also significantly decreased caudal epididymal sperm counts (p<0.01). Some of motion kinematic parameters (motility, mean angular displacement, lateral head displacement, and dance) of sperms retrieved from vasa deference were significantly decreased in rats treated with the TBT chloride at the dose of 20 mg/kg B.W. (p<0.05). These results provide a further evidence that an exposure to TBT chloride during pubertal period in male rats produces spermatogenic disorders characterized by decreasing testicular and epididymal sperm counts and some motion parameters of sperms in the vasa deference. KEY WORDS: epididymis, sperm count, sperm motility, testis, tributyltin chloride.J. Vet. Med. Sci. 65(12): 1331-1335, 2003 Tributyltin (TBT) has been widely used as a biocide in such applications as antifouling paints of boat and for other purposes, until its use restricted in 1980's after discovery of severe damage on aquatic ecosystem caused by the agent [4,11,16]. Even after the restriction of its uses, TBT deposited in sediments remains an important exposure source for marine biota many years to come due to its relative stability under anoxic sediment conditions [4,10,11]. The bioaccumulation of TBT and its metabolites (dibutyltin and monobutyltin) in food chains and food products has lately attracted a considerable attention on human health effects [3,15,17]. Though the levels of TBT compounds were not sufficiently high to have adverse effects on human health, a possible exposure of humans to TBT compounds aroused a great concern about their toxic potential [17].A variety of reproductive toxicities of TBT compounds have been reported in rats [1,2,6,7,12,13]. The exposure of TBT chloride during preimplantation period produced early embryo loss and implantation failure in rats [6,7]. In addition, TBT chloride exposure during pregnancy has been associated with increased incidence of fetuses with cleft palate and induced fetal reabsorption in rats [1,2]. In the study of two-generation reproductive toxicity in the rat, decreases in body weight and sex organ weights were pronounced, and sperm counts of testis and cauda epididymis were also decreased in F1 and F2 neonates [12,13].In our previous report, TBT treatment in male rats during puberty decreased the weights of prostate and seminal vesicle, and increased detached debris and some sloughed cells rega...